This invention relates to printing processes, and more particularly, to methods for determining feature-size accuracy in photolithographic printing.
Photolithographic printing is widely used in the electronics industry for defining electronic circuit patterns on semiconductor wafer surfaces. A large scale version of the pattern is typically made which is photographically reduced in size to form the mask used in printing the circuit pattern. The pattern is defined by transparent regions on the mask through which light is projected for exposure of a photoresist film on the wafer surface.
As electronic circuit patterns have become more complex, it has become important to monitor the accuracy with which the patterns are printed, particularly in the production of masks. Overexposure during photolithographic printing, for example, may result in feature sizes slightly larger than intended. Since accuracy requirements are typically less than one micron, it is important to develop techniques for determining any such deviations.
Because the desired circuit patterns are defined by opaque and transparent areas, devices such as the scanning densitometer, which record light transmission as a function of distance, may be used for monitoring feature size accuracy. This of course requires sophisticated equipment and is cumbersome and time-consuming to use.